Alligator,
Yep, that's a vacuum breaker. It is designed to keep a siphon from ever happening. Looking at your new and improved drawing, it appears that you might get a siphon going just as the water starts flowing over the top of your dam, but with the amount of vacuum a 12" siphon can pull, you would probably get a nice whirlpool over the 4" pipe that would easily suck in enough air to break the siphon. Even if the 4" pipe is a foot under water, a whirlpool can suck in a lot of air. When this happens, the siphon will start, then it will suck in air and stop, then it will start again, then stop, and so on.

I design large aquarium systems that use the same siphon system we have been discussing here. Most use a 1 1/2 inch siphon tube with a 1/2" vent. With this I can get a whirlpool to suck air down 3-4 inches below the surface. You can only imagine what a 12" siphon with a 4" vent can do. A 12" pipe is 64 times as big as a 1 1/2 inch pipe.

Your system would be very easy to convert to a real siphon system. If you want to wait and see what it does, I just hope you have a well built emergency spillway. If not, get yourself some kind of cap to temporarily seal off the vent if the water gets too high. Just remember to remove the cap before your pond is empty!

Hey Moe, I'm trying to think but nuthin's happening!